1. Field of the Invention
The invention relates to the preparation and the application for a suspension that contains the nano-scale electrolyte of YSZ (yttria stabilized zirconia)/GDC (Gd doped ceria)/LSGM (strontium and magnesium doped lanthana gallat) and is used to produce membrane electrode assembly (MEA) of solid oxide fuel cells (SOFC). Through spin coating technology, solid oxide fuel cell-membrane electrode assembly with airtight/fully dense electrolyte layers can be produced to possess high electrochemical property, durability and high degradation resistance. The attributes of the manufacturing process include low cost, high reliability and mass production capability.
2. Description of the Prior Art
With rising oil price and growing consciousness of environmental protection, renewable energy technology is one of the most important technologies in the century. Solid oxide fuel cell is a power generation system with high efficiency, low pollution and diversified energy source. It has become the power generation system that has the most development potential because its features like simple material composition, modulized structure and stable and sustainable power generation. Among all, planar solid oxide fuel cells can overcome long circuit loss and exhibit uniform current collection and therefore can increase cell power density. Planar solid oxide fuel cells are the primary target of research and development for many research groups presently.
The raw materials for solid oxide fuel cells are all ceramic powders of solid oxides. Taking Anode Supported Cell (ASC) as an example, the primary cell type under research uses NiO+YSZ as anode material, YSZ as electrolyte material, and LSM/LSCF as cathode material. Many literatures indicate that because nano-scale ceramic powders have high specific surface area to achieve fully dense ceramic structure under a low sintering temperature. In the manufacturing process for solid oxide fuel cells, low sintering temperature can lower cost, improve planarity of anode supported cell substrate, decrease growth of nickel crystal grains, effectively increase solid oxide fuel cell performance and lower production cost.
Many literatures have proposed diverse production technologies for nano-scale powders, such as gas-phase combustion, spray-drying, sol-gel and hydrothermal. The hydrothermal technology has many advantages in the preparation of nano-scale ceramic powders because it does not use organic solvents, does not need additional sintering process, have excellent control of chemical composition and possess continuous mass production capability.
Currently in a typical SOFC-MEA production process, the anode supported cell substrate is fabricated by tape casting technique first for green tape and then completed by high temperature calcination/sintering. Cathode layer is usually prepared by screen printing technique. Most of the anode supported cell substrates and cathode layers have porous structure. To increase cell performance and lower operation temperature, YSZ electrolyte layer needs airtight/fully dense membranes. Thus, it is very important to develop a technique to coat electrolyte layers onto a porous anode substrate.
Siemens-Westinghouse uses electrochemical vapor deposition (EVD) technique to prepare airtight/fully dense electrolyte layers and successfully apply them to tubular SOFC system. However, this technique needs special equipment and process, so its production cost is high. Alternative techniques have been proposed, such as Plasma-Spray, Sputtering Coating, Sol-Gel, and Spin Coating etc. Among these, Plasma-Spray and Sputtering Coating still have high production cost. Sol-Gel technique has difficulties in preparing fully dense layers onto a porous substrate and would increase defect rate and quality control cost. Spin Coating technique has low equipment cost, simple production process and high reliability, so it has many advantages in preparing airtight/fully dense electrolyte layers.
Presently the key technique for spin coating is how to obtain a well-dispersed suspension as coating material. Nano-scale ceramic powders have small size, high surface energy and tendency to agglomerate during preparation, storage and use. Agglomeration in ceramic powders is disadvantageous to uniformity and densification, which would lead to additional crystal grain growth to induce sintering stress or pore formation that is harmful to the preparation of airtight/fully dense electrolyte layers.
In a suspension, agglomeration in nano-scale powders occurs due to inter-particle Van der Waals attractive forces, but can be overcome by inter-particle electrostatic repulsive forces. The particle electrostatic field is zeta potential, which can be adjusted by the pH value of suspension. The higher zeta potential is the higher electrostatic repulsive force is. Although at an extreme pH value (high acidity or high basicity) there is high zeta potential, the ionic strength of suspension is also high and therefore particles are close to each other and agglomeration can occur.
Another method is to increase inter-particle stereo-hindrance to achieve dispersion of nano-scale ceramic powders. This method is to add suitable polymer dispersant in the suspension, so the polymer dispersant will completely cover the particle surface and prevent agglomeration due to inter-particle contact.
Hence, it is required that the membrane from a spin coating process does not crack during drying. Thus, the suspension needs a suitable binder, such as polyvinyl alcohol (PVA) and methyl cellulose water-soluble type binder etc.
Therefore, the invention proposes a formulation and a process for a nano-scale electrolyte (YSZ/GDC/LSGM) suspension, and the application of the suspension by spin coating technique to prepare airtight/fully dense electrolyte layers with advantages like low production cost, high reliability and mass production capability.